Digital radiography offers the possibility for computer aided diagnosis and quantitative analysis using image processing techniques such as segmentation, classification and contrast enhancement. However, computer-based image interpretation may be hindered by the presence of the non-uniformities in radiation exposure that are inherent to the image formation.
In the field of X-ray exposure these non-uniformities can be largely attributed to the Heel effect, nevertheless other sources of inhomogeneities exist such as recording member non-uniformities or read-out inhomogeneities.
Although the intensity inhomogeneities induced by all these factors are smoothly varying as a function of location and are easily corrected by the human visual perception system, they complicate the use of automatic processing techniques because the relative brightness of an object within the image becomes position dependent. The overall intensity range is unnecessarily enlarged by the presence of these slowly varying shading components and hence the dynamic range available to represent diagnostic signal details is reduced.
A typical hand radiograph is shown in FIG. 1. The background at the left side of the image is clearly brighter than at the right side. This phenomenon can be attributed to the so-called Heel effect. Because beam collimator blades substantially attenuate the X-ray beam so as to shield irrelevant body parts, the phenomenon is only visible in the direct exposure and diagnostic areas and not in the collimation area. The Heel effect can be understood from the construction of the X-ray tube as schematically depicted in FIG. 2. Electrons originating from the cathode are attracted by the positively charged anode. For better heat dissipation, the anode rotates and is inclined by a small anode angle θ, which enlarges the area Aactual that is bombarded by electrons while keeping the size of the focal Aeff, from which rays are projected downward to the object, fairly small. As shown in the diagram of FIG. 3, this design makes the length of the path travelled by the X-rays through the anode larger on the anode side of the field Ta than on the cathode side Tc. Hence the incident X-ray intensity is smaller at the anode side than at the cathode side of the recording device, which explains the inhomogeneity of the background in FIG. 1.
The Heel effect is one possible cause of intensity inhomogeneities that can be introduced in radiographs. As has already been mentioned higher, other causes of non-uniformities might be envisioned such as non-uniform sensitivity of the recording member, e.g. a photographic film, a photostimulable phosphor screen, a needle phosphor screen, a direct radiography detector or the like. Still another cause might be non-uniformities of the read-out system which is used for reading an image that has been stored in a recording member of the kind described above.
Because the image acquisition parameters that affect intensity inhomogeneity vary from image to image (e.g. variable positioning of the recording device relative to the X-ray source) and can not be recovered from the acquired image at read-out, correction methods based on calibration images or flat field exposure such as the one described in EP-A-823 691 are not feasible.
The method disclosed in EP-A-823 691 comprises the steps of (1) exposing an object to radiation emitted by a source of radiation, (2) recording a radiation image of said object on a radiation-sensitive recording member, (3) reading the image that has been stored in said recording member and converting the read image into a digital image representation, (4) generating a set of correction data and (5) correcting said digital image representation by means of said set of correction data. The set of correction data is deduced from data corresponding with a uniform, flat field exposure of the recording member. The set of correction values represent the deviation of the value that is effectively obtained in a pixel of the recording member and a value that would be expected in the case of flat field exposure. These correction values associated with each recording member are normally determined once and kept fixed during further acquisition cycles.
This type of methods is not applicable for solving problems such as the introduction of inhomogeneities due to the Heel effect.